Guys: Does it mean you're losing interest when you don't text back?

My boyfriend who I've been in a semi- long distance relationship with, (we're an hour apart and only get to see each other once a month) has all of a sudden stopped texting me back. Usually he'll text me back with in 30 minutes or an hour. But lately I've been lucky to get a reply within 12 hours of my message. He never has a reason why he hasn't texted back, and he's been responding with shorter and shorter messages. He's been finding excuses to not Skype (or to cut our Skype calls shorter). Is he losing interest? (Btw these new behaviors have been going on only recently, the past week or so). Please tell me I'm over reacting! Or do you think he may be losing interest? Please help!